The present invention relates to magnet cores for power transformers and particularly to a method for facilitating the construction of a rectangular, laminated magnetic core having stepped-lap joints between the leg and yoke portions of the core.
Stepped-lap joints in laminated magnetic cores are well known and have been formed to promote reduced core excitation current requirements, lower core losses, and decreased noise levels. Examples of stepped-lap joint cores are shown in U.S. Pat. Nos. 2,628,273, 3,153,215, 3,210,709, 3,477,053, 3,540,120 and 3,670,279. To simplify the manufacturing process, the leg and yoke laminations are typically cut to equal lengths; their ends being mitered. Upon assembly of the core, the leg and yoke portions of the core have their laminations in stacked relation with the midpoints incrementally, longitudinally offset. This creates a stepped relationship of the lamination ends at each end of any stacked group of leg and yoke laminations. Thus for any stacked group of laminations, the stepped ends face in opposite directions at the ends of the group. When the leg and yoke laminations are assembled individually to build up the core a layer at a time, assembly is straightforward, but very time consuming. To save time, it has been proposed to build up the core by assembling the leg and yoke laminations in groups of laminations. However, this poses a problem when one group of laminations is to be assembled with two other lamination groups, requiring the mating of two stepped-lap joints. Since the stepped ends of the one lamination group face in opposite directions, one stepped end must be tucked under a stepped end of one of the other groups--analogous to manipulating the last of the four flaps to close the end of a cardboard carton in sustained over and under relation.
This problem was addressed in the above-noted U.S. Pat. No. 3,670,279. To eliminate so-called "blind" stepped-lap joint assembly situations, the laminations are arranged in stacks of leg and yoke groups, with the laminations of each group being of incrementally different lengths. With the longitudinal midpoints of the laminations aligned and the laminations stacked according to length progressing from the shortest to the longest, the stepped ends of the laminations in any group face in the same direction. Thus, for example, a pair of leg lamination groups may be positioned in spaced parallel relation with their stepped ends faced upwardly, and then a pair of yoke lamination groups, oriented with their stepped ends faced downward, could then be simply set into place to complete the stepped-lap joints at the four corners. The principal drawback to this approach is that so many different lamination lengths are required, rendering the cutting and grouping operations more difficult and expensive.